1. Field of the Invention
The present invention relates to a wire electric discharge machine, and particularly to temperature control of machining fluid being used in the wire electric discharge machine.
2. Description of Related Art
The wire electric discharge machine machines a workpiece by producing electric discharges by applying a voltage between a wire electrode and the workpiece. For insulation between the wire electrode and the workpiece, cooling, and removal of swarf produced by electric discharges, it is arranged such that machining fluid is intervened between the wire electrode and the workpiece. The electric discharges between the wire electrode and the workpiece heat the machining fluid, namely cause a rise in temperature of the machining fluid. Also heat loss of a pump for supplying the machining fluid to a machining tank with the workpiece placed in, etc. causes a rise in temperature of the machining fluid.
This temperature rise of the machining fluid causes thermal expansion deformation of the workpiece, a table on which the workpiece is mounted, etc., which causes lowering of the machining accuracy and breaking of the wire electrode. Thus, the wire electric discharge machine is provided with a cooling device for cooling the machining fluid.
FIG. 6 is a schematic block diagram showing a function of controlling the temperature of the machining fluid on the basis of a machining-fluid temperature in a clean-fluid tank, adopted in a conventional wire electric discharge machine.
In FIG. 6, reference numeral 1 denotes mechanical unit of the wire electric discharge machine. A table on which a workpiece to be machined is mounted is placed in a machining tank 2 disposed on the mechanical unit 1, and a wire electrode (not shown) is arranged to run within the machining tank 2. Electric discharge machining is performed by moving the table by means of feed servomotors, etc. included in the mechanical unit 1 and producing electric discharges by applying a voltage between the workpiece and the wire electrode. The machining tank 2 stores machining fluid. The machining fluid heated by electric discharges and containing swarf flows out of the machining tank 2 and is collected and held in a contaminated-fluid tank 3. The machining fluid in the contaminated-fluid tank 3 is pumped up by a pump P1 provided for filtering, passed through a filter F, and supplied to and held in a clean-fluid tank 4.
The machining fluid in the clean-fluid tank 4 is pumped up through a machining-fluid supply line L1a by a pump P3 provided for circulation. The machining-fluid supply line branches at the discharge port of the pump P3 so that the machining fluid is supplied to the machining tank 2 through a machining-fluid supply line L1b, and also introduced to a machining-fluid cooling device 6. To the clean-fluid tank 4 or the machining-fluid cooling device 6, a temperature sensor S1 for detecting the temperature of the machining fluid in the clean-fluid tank 4 is provided. On the basis of the temperature detected by the temperature sensor S1, the machining-fluid cooling device 6 performs temperature control to cool the machining fluid to a determined temperature. The machining fluid cooled is returned to the clean-fluid tank 4 through a machining-fluid return line L1c. 
Further, a pump P2 provided for spouting pumps up the machining fluid from the clean-fluid tank 4 through a machining-fluid supply line L2a, and supplies it to upper and lower wire guides 5 through a machining-fluid supply line L2b so that the machining fluid is spouted from nozzles provided to the respective wire guides 5 to a space between the workpiece and the wire electrode (see JP 8-174339A).
There is also known machining-fluid treatment in which the machining-fluid temperature is controlled on the basis of the machining-fluid temperature in the machining tank, as shown in FIG. 7. A temperature sensor S2 for detecting the temperature of the machining fluid in the machining tank 2 is provided to the machining tank 2, and the machining-fluid temperature in the clean-fluid tank 4 is controlled by pumping up the machining fluid from the clean-fluid tank 4 by means of a pump P3, subjecting it to cooling control by the machining-fluid cooling device 6, on the basis of the temperature detected by the temperature sensor S2, and returning it to the clean-fluid tank 4. In the other respects, the configuration is the same as that of the example shown in FIG. 6 (see JP 63-120038A).
In the conventional example shown in FIG. 6, the machining fluid held in the clean-fluid tank 4 is controlled to a determined temperature by the machining-fluid cooling device 6. The machining fluid is, however, supplied to the machining tank 2 and the upper and lower guides 5 (note that the machining fluid is supplied to a space between the wire electrode and the workpiece, via upper and lower wire guides 5), through the pump P3 for circulation and the pump P2 for spouting. Thus, the machining fluid supplied is at a temperature raised due to heat loss of the pumps P3 and P2. Further, the machining fluid in the machining tank 2 is heated by electric discharges between the wire electrode and the workpiece, so that it is at a raised temperature, or in other words, uncontrolled in temperature. Thus, the temperature of the machining fluid in the machining tank 2 is higher than the temperature of the machining fluid in the clean-fluid tank 4 which is controlled in temperature, and the temperature in the machining tank 2 varies depending on the state of machining.
FIG. 8 shows how the machining-fluid temperature in the machining tank varies in rough machining and finish machining, under the machining-fluid temperature control in the conventional example shown in FIG. 6. In FIG. 8, time is plotted on the horizontal axis and the machining-fluid temperature in the machining tank is plotted on the vertical axis. The target temperature for the machining-fluid temperature control by the machining-fluid cooling device 6 is indicated in dashed line. In the rough machining, the amount of heat produced by machining and heat due to the pumps is great, so that the machining-fluid temperature in the machining tank 2 is higher than the target temperature, although the machining-fluid temperature in the clean-fluid tank 4 is controlled to the target temperature by the machining-fluid cooling device 6. Meanwhile, in the finish machining, heat is hardly produced by machining and heat due to the pumps is little, so that a difference between the machining-fluid temperature in the machining tank 2 and that in the clean-fluid tank 4 is very small, or in other words, the machining-fluid temperature in the machining tank 2 is controlled almost to the target temperature. This leads to a drawback that there is produced a step in machining-fluid temperature in the machining tank, between the rough machining and the finish machining.
In the machining-fluid temperature control system in the conventional example shown in FIG. 7, the machining-fluid temperature in the machining tank 2 is detected and controlled to follow the target temperature. Thus, the problem of influence of machining heat and heat due to the pumps P2, P3 is obviated, and the machining-fluid temperature control can be performed uniformly from rough machining to finish machining, in spite of a change in machining state. There is, however, a problem that in the finish machining, the band of variation of the temperature is broad.
FIG. 9 shows how the machining-fluid temperature detected in the machining tank 2 varies in rough machining and finish machining, in the system shown in FIG. 7 in which the machining-fluid temperature in the machining tank 2 is detected and the machining-fluid temperature in the clean-fluid tank 4 is cooling-controlled by the machining-fluid cooling device 6. As seen from FIG. 9, in the finish machining, the band of variation of the machining-fluid temperature in the machining tank 2 is broader, compared with the control system shown in FIG. 6 (compared with the variation shown in FIG. 8).
The cause of this phenomenon is thought to be that the machining-fluid temperature in the machining tank 2 is controlled by detecting the temperature in the machining tank 2, cooling the machining fluid in the clean-fluid tank 4 and supplying this cooled machining fluid to the machining tank.
In the rough machining, much heat is produced by machining, so that the machining-fluid temperature in the machining tank 2 rises at a high rate. When the machining-fluid temperature in the machining tank 2 detected by the temperature sensor S2 rises beyond the target temperature for the temperature control by the machining-fluid cooling device 6, to a cooling start temperature, the machining-fluid cooling device 6 starts cooling the machining fluid in the clean-fluid tank 4. The machining fluid pumped up from the clean-fluid tank 4 is cooled, then returned to the clean-fluid tank 4 and mixed, so that the machining-fluid temperature gradually drops. The machining fluid gradually dropping in temperature is put into the machining tank 2, and due to a temperature difference between the machining fluid in the machining tank 2 and the machining fluid in the clean-fluid tank 4, the machining-fluid temperature in the machining tank 2 drops. Then, when the machining-fluid temperature detected by the temperature sensor S2 drops below the target temperature to the lower limit of the temperature control, the temperature control by the machining-fluid cooling device 6 is deactivated, so that the cooling of the machining fluid in the clean-fluid tank 4 is stopped. Thus, after this, the machining fluid fixed in temperature is put into the machining tank 2. This supply of the machining fluid fixed in temperature tends to drop the machining-fluid temperature in the machining tank 2, while the heat produced by machining warms the machining fluid. Thus, when the rate at which the machining-fluid temperature rises due to the heat produced by machining exceeds the rate at which the temperature drops due to the machining fluid from the clean-fluid tank 4, the machining-fluid temperature starts rising. This process is repeated, so that the machining-fluid temperature is held close to the target temperature, repeating a rise and drop with a short period, as shown in FIG. 9.
Meanwhile, in the finish machining, machining produces little heat. Thus, the machining-fluid cooling device 6 starts cooling the machining fluid in the clean-fluid tank 4, namely causes its temperature to drop gradually, the machining fluid dropped in temperature is put into the machining tank 2, and due to a temperature difference between the machining fluid in the machining tank and the machining fluid in the clean-fluid tank 4, also the machining-fluid temperature in the machining tank 2 drops. Then, when the machining-fluid temperature that has reached the lower limit of the temperature control is detected by the temperature sensor S2, the machining-fluid cooling operation of the machining-fluid cooling device 6 is stopped, so that the machining-fluid temperature in the clean-fluid tank 4 stops dropping and held at a fixed temperature. Since this machining fluid in the clean-fluid tank 4 is put into the machining tank 2, the machining-fluid temperature in the machining tank 2 still continues dropping, due to a temperature difference between the machining fluid in the machining tank 2 and the machining fluid in the clean-fluid tank 4. Since in the finish machining, machining produces little heat, the machining-fluid temperature in the machining tank 2 drops to almost the same level as the machining-fluid temperature in the clean-fluid tank 4, and accordingly, the rate at which the machining-fluid temperature in the machining tank drops due to the temperature difference between the machining fluid in the clean-fluid tank 4 and the machining fluid in the machining tank 2 decreases. When the rate at which the machining-fluid temperature rises due to the heat produced by finish machining exceeds this temperature drop rate, the machining-fluid temperature in the machining tank 2 rises. In the finish machining, this process is repeated. Thus, as seen from FIG. 9, the cycle of rise and drop of the machining-fluid temperature in the machining tank 2 is longer, the band of variation of the temperature is broader, and there is produced a step in temperature between the rough machining and the finish machining. In other words, in the finish machining, the average of the machining-fluid temperature in the machining tank is lower than the target temperature for the machining-fluid cooling device 6 and different from that in the rough machining. Further, in the finish machining, the band of variation of the temperature is broad, which means low stability.